This invention relates to radio broadcasting and, more particularly, to methods of and apparatus for demodulating and equalizing a signal in a receiver for an amplitude modulated compatible digital broadcasting system.
There has been increasing interest in the possibility of broadcasting digitally encoded audio signals to provide improved audio fidelity. Several approaches have been suggested. One such approach, set forth in U.S. Pat. No. 5,588,022, teaches a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. The broadcast signal includes an amplitude modulated radio frequency signal having a first frequency spectrum. The amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal. The signal also includes a plurality of digitally modulated carrier signals within a bandwidth which encompasses the first frequency spectrum. Each of the digitally modulated carrier signals is modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal.
The waveform in the AM compatible digital audio broadcasting system described in U.S. Pat. No. 5,588,022, was formulated to provide optimal data throughput for the digital signal while avoiding crosstalk into the analog AM channel. Multiple carriers are employed by means of orthogonal frequency division multiplexing (OFDM) to bear the communicated information.
Monophonic detectors for consumer AM radios respond only to the envelope and not the phase of the received signal. Because of the use of the multiple digitally modulated carriers, there is a need for a means to reduce the envelope distortion caused by this hybrid signal. U.S. patent application Ser. No. 08/671,252, assigned to the assignee of the present invention, discloses a method for reducing envelope distortion in an AM compatible digital audio broadcasting system. Certain digital carriers that are above the frequency of the analog AM carrier have an associated digital carrier that is at an equal frequency offset below the analog AM carrier. The data and modulation placed on the upper digital carrier and its counterpart are such that the signal resulting from their addition has no component that is in-phase with the analog AM carrier. Digital carrier pairs arranged in this way are said to be complementary. Carriers that are not directly beneath the analog signal spectrum are called non-complementary, and are modulated in-phase and in-quadrature to the AM carrier. This configuration delivers dramatic fidelity improvements to analog AM reception of digital broadcast signals.
At the receiver, the digital signal is demodulated by means of a Fast Fourier Transform (FFT). One possible method and associated apparatus to perform the demodulation is described in U.S. Pat. No. 5,633,896. That patent discloses a demodulation technique which minimizes the undesired crosstalk between the analog signal and the digital signals in an AM compatible digital audio broadcasting (AM DAB) system using an orthogonal frequency division multiplexed (OFDM) modulation format, by employing dual Fast Fourier Transform processes on separate respective in-phase and quadrature-phase components of a received OFDM digital signal. The output of the quadrature channel is used to recover the complementary data, and the resultant processed component signals are summed to recover the non-complementary data.
The received multi-carrier signal requires equalization in the presence of dynamic channel response variations. Without such equalization, a very distorted signal would be detected and the digital broadcasting signal information would be unrecoverable. An equalizer enhances the recoverability of the digital audio broadcasting signal information. One such equalizer is disclosed in U.S. Pat. No. 5,559,830. The equalizer disclosed therein includes means for receiving an AM compatible digital audio broadcasting waveform and storing that waveform as a waveform vector. The equalizer then processes that waveform by multiplying the waveform vector by an equalization vector. This equalization vector comprises a plurality of equalizer coefficients, each of the coefficients initially set to a predetermined value. The equalizer then compares each location of the processed waveform vector with a stored waveform vector. The equalizer selects as the signal that vector location closest to the stored waveform vector. Preferably, the equalizer includes means for updating the equalizer coefficients using the waveform vector, the processed waveform vector, and the stored waveform vector to provide immunity to noise and response to channel changes.
In the equalizers of both U.S. Pat. No. 5,633,896 and U.S. Pat. No. 5,559,830, frequency domain information is presented to the equalizer as a frequency domain vector. Each block of frequency domain information is stored in a storage array. This storage array vector is multiplied by a plurality of equalizer coefficients. The resulting product of this multiplication is an equalized signal. A set of exact values is known a priori in the equalizer against which each vector location of the equalized signal can be compared. The ideal value closest to that described in the vector location is chosen as the actual signal value. The vector of decisions is stored in a decision array. Using the received signal, the equalized signal, and the decision array, an equalizer coefficient estimator calculates coefficient estimates. To provide immunity to noise, the equalizer coefficient estimates can be averaged over time. The rate of coefficient update determines equalizer noise immunity and convergence rate. Coefficients in different parts of the band may be updated at different rates depending on knowledge of the distortion mechanism. U.S. Pat. No. 5,633,896 and U.S. Pat. No. 5,559,830 are hereby incorporated by reference.
While the dual FFT technique can improve system performance in a channel that has symmetric magnitude and anti-symmetric phase about the AM carrier frequency over the frequency range of the complementary carriers, for channels with non-symmetric magnitude or non- anti-symmetric phase, the process of using only the quadrature channel FFT output to obtain the complementary data destroys the non-symmetric magnitude and non- anti-symmetric phase information and the signal that drives the equalizer is not correct. A United States patent application for a "Method For Equalization Of Complementary Carriers In An AM Compatible Digital Audio Broadcast System", by the present inventors and M. Matherne, filed on the same date as this application, and assigned to the same assignee, discloses an equalization method that can provide proper equalizer coefficients when the equalizer coefficients may have non-symmetric magnitude or non- anti-symmetric phase.
Demodulation of the non-complementary carriers may require a high-pass filter on the in-phase portion of the signal to eliminate spectral spillage in the FFT from the analog signal. However, when a high-pass filter is applied, information in the in-phase signal is destroyed, thus preventing proper equalization of the complementary digital carriers. For channels that have non-symmetric magnitude or non- anti-symmetric phase over the spectral region of the analog signal, the destroyed information prevents proper equalization of the complementary carriers. The channel, as referred to here, includes not only phenomenon that affect propagation of the signal, but also any component in the transmitter or receiver that affects the magnitude and phase of the received signal. The present invention provides a method of demodulating the digital signal without the drawback of either spectral spillage of the analog signal onto the non-complementary carriers or destroying information needed for proper equalization of the complementary carriers. The present invention seeks to provide an improved demodulation and equalization method and receivers which include the method.